Graphene oxide gel bonded graphene composite films and processes for producing same

ABSTRACT

Disclosed is a graphene composite thin film composition composed of nano graphene platelets (NGPs) bonded by a graphene oxide binder, wherein the NGPs contain single-layer graphene or multi-layer graphene sheets having a thickness from 0.335 nm to 100 nm. The NGPs occupy a weight fraction of 1% to 99.9% of the total composite weight. The graphene oxide binder, having an oxygen content of 1-40% (preferably &lt;10%) by weight based on the total graphene oxide weight, is obtained from a graphene oxide gel. The composite forms a thin film with a thickness no greater than 1 mm, but preferably no greater than 100 μm and no less than 10 μm. This composition has a combination of exceptional thermal conductivity, electrical conductivity, and mechanical strength unmatched by any thin-film material of comparable thickness range.

This invention is based on the results of a research project sponsored by the US National Science Foundation SBIR-STTR Program.

FIELD OF THE INVENTION

The present invention relates generally to the field of graphitic materials for heat dissipation applications, and more particularly to graphene composite films with an exceptionally high thermal conductivity, high electrical conductivity, and high mechanical strength.

BACKGROUND OF THE INVENTION

Carbon is known to have five unique crystalline structures, including diamond, fullerene (0-D nano graphitic material), carbon nano-tube (1-D nano graphitic material), graphene (2-D nano graphitic material), and graphite (3-D graphitic material).

The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. Carbon nano-tubes have a diameter on the order of a few nanometers to a few hundred nanometers. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material.

A graphene sheet is composed of carbon atoms occupying a two-dimensional hexagonal lattice. An isolated, individual graphene sheet is commonly referred to as single-layer graphene. A stack of multiple graphene planes bonded through van der Waals forces in the thickness direction is commonly referred to as a multi-layer graphene, typically having 2-300 layers or graphene planes, but more typically 2-100 graphene planes. Single-layer graphene and multi-layer graphene sheets are collectively called “nano-scaled graphene platelets (NGPs).” Our research group pioneered the development of graphene materials and related production processes: (1) B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006), application submitted in October 2012]; (2) B. Z. Jang, et al. “Process for Producing Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004); and (3) Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25, 2006).

NGPs are typically obtained by intercalating natural graphite flakes with a strong acid and/or oxidizing agent to obtain a graphite intercalation compound (GIC), as illustrated in FIG. 1. This is most often accomplished by immersing natural graphite flakes in a mixture of sulfuric acid, nitric acid (an oxidizing agent), and another oxidizing agent (e.g. potassium permanganate or sodium chlorate). The resulting WC is actually some type of graphite oxide particles. This GIC is then repeatedly washed and rinsed in water to remove excess acids, resulting in a graphite oxide suspension or dispersion which contains discrete and visually discernible graphite oxide particles dispersed in water. There are two processing routes to follow after this rinsing step:

Route 1 involves removing water from the suspension to obtain “expandable graphite,” which is essentially dried GIC or dried graphite oxide particles. Upon exposure of expandable graphite to a temperature in the range of 800-1050° C. for approximately 30 seconds to 2 minutes, the GIC expands by a factor of 30-300 to form a “graphite worm,” which is a collection of exfoliated, but largely unseparated or interconnected graphite flakes. In Route 1A, these graphite worms (exfoliated graphite or “networks of interconnected/non-separated graphite flakes”) can be re-compressed to obtain flexible graphite sheets that typically have a thickness in the range of 0.125 mm (125 μm)-0.5 mm (500 μm). These flexible graphite (FG) sheets are used as a seal material and a heat spreader material, but exhibiting a maximum in-plane thermal conductivity of up to 600 W/mK (typically <300 W/mK) and in-plane electrical conductivity no greater than 1,500 S/cm. In Route 1B, the exfoliated graphite is subjected to mechanical shearing (e.g. using an ultrasonicator, high-shear mixer, air jet mill, or ball mill) to form separated single-layer and multi-layer graphene sheets (collectively, NGPs), as disclosed in our U.S. application Ser. No. 10/858,814.

Route 2 entails ultrasonicating the graphite oxide suspension for the purpose of separating individual graphene oxide sheets from graphite oxide particles. This is based on the notion that the inter-graphene plane separation has been increased from 0.335 nm in natural graphite to 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakening the van der Waals forces that hold neighboring planes together. Ultrasonic power can be sufficient to further separate graphene plane sheets to form separate, isolated, or discrete graphene oxide (GO) sheets. These graphene oxide sheets can then be chemically or thermally reduced to obtain “reduced graphene oxides” (RGO) typically having an oxygen content of 2-10% by weight, more typically 25% by weight.

For the purpose of defining the claims of the instant application, NGPs include single-layer and multi-layer graphene or reduced graphene oxide with an oxygen content of 0-10% by weight, more typically 0-5% by weight, and preferably 0-2% by weight. Pristine graphene has essentially 0% oxygen.

It may be noted that flexible graphite sheets (obtained by re-compression of exfoliated graphite or graphite worms) exhibit a relatively low thermal conductivity (<600 W/mK as recited above). Flexible graphite sheets are also of low strength and poor structural integrity. The high tendency for flexible graphite sheets to get torn apart makes them difficult to handle in the process of integrating them in a microelectronic device.

Similarly, the NGPs, when packed into a film or paper sheet of non-woven aggregates, exhibit a thermal conductivity higher than 1,000 W/mK only when the film or paper is cast and pressed into a sheet having a thickness lower than 10 μm, and higher than 1,500 W/mK only when the film or paper is cast and pressed into a sheet having a thickness lower than 1 μm. This is reported in our earlier U.S. patent application Ser. No. 11/784,606 (Apr. 9, 2007). However, ultra-thin film or paper sheets (<10 μm) are difficult to produce in mass quantities, and difficult to handle when one tries to incorporate these thin films as a heat spreader material during the manufacturing of microelectronic devices. Further, thickness dependence of thermal conductivity (not being able to achieve a high thermal conductivity at a wide range of film thicknesses) is not a desirable feature.

Our earlier application (Ser. No. 11/784,606) further disclosed a mat, film, or paper of NGPs infiltrated with metal, glass, ceramic, resin, and CVD graphite matrix material. Later on, Haddon, et al (US Pub. No. 2010/0140792, Jun. 10, 2010) also reported NGP thin film and NGP-polymer composites for thermal management applications. The processes used by Haddon et al to produce NGPs are identical to those disclosed much earlier by us (Jang, et al. U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004)). Balandin, et al (US Pub. No. 2010/0085713, Apr. 8, 2010) also disclosed a graphene layer produced by CVD deposition or diamond conversion for heat spreader application. More recently, Kim, et al (N. P. Kim and J. P. Huang, “Graphene Nanoplatelet Metal Matrix,” US Pub. No. 2011/0108978, May 10, 2011) reported metal matrix infiltrated NGPs. However, metal matrix material is too heavy and the resulting metal matrix composite does not exhibit a high thermal conductivity.

Another prior art material for thermal management application is the pyrolitic graphite film. The lower portion of FIG. 1 illustrates a process for producing prior art graphitic films or sheets. The process begins with carbonizing a polymer 46 at a carbonization temperature of 500-1,000° C. for 2-10 hours to obtain a carbonized material 48, which is followed by a graphitization treatment at 2,500-3,200° C. for 5-24 hours to form a graphitic film 50. This is a slow, tedious, and energy-intensive process. Carbonization of certain polymers (e.g. polyacrylonitrile) involves the emission of toxic species.

Thus, it is an object of the present invention to provide a highly conductive GO gel-bonded NGP composite thin-film structure (and related production processes) that exhibits a thermal conductivity greater than 600 W/mK, typically greater than 800 W/mK, more typically greater than 1,500 W/mK (even when the film thickness is greater than 10 μm), and most preferably and often greater than 1,700 W/mK.

It is another object of the present invention to provide an NGP-GO composite thin-film sheet that exhibits a relatively thickness-independent thermal conductivity.

Still another object of the present invention is to provide a GO-bonded pristine graphene composite thin film that exhibits exceptional thermal and electrical conductivity properties.

It is a further object of the present invention to provide an NGP-GO composite thin-film sheet that is lightweight and exhibits a relatively high strength or structural integrity.

It is yet another object of the present invention to provide a highly conductive NGP-GO composite thin-film sheet (and related processes) wherein the in-plane thermal conductivity is greater than 600 W/mK (preferably and typically greater than 1,000 W/mK) and in plane electrical conductivity is greater than 2,000 S/cm (preferably and typically >3,000 S/cm), and/or a tensile strength greater than 10 MPa (preferably and typically >40 MPa).

It is another object of the present invention to provide a highly thermally conductive NGP-GO composite thin-film sheet that can be used for thermal management applications; e.g. for use as a heat spreader in a microelectronic device (such as mobile phone, notebook computer, and tablet), flexible display, light-emitting diode (LED), power tool, computer CPU, and power electronics. We are filing separate patent applications to claim the various products or applications of the presently invented NGP-GO composite thin-films.

SUMMARY OF THE INVENTION

The present invention provides a graphene composite thin film composition composed of nano graphene platelets (NGPs) bonded by a graphene oxide binder. There is no other resin binder or matrix material involved or included in this graphene composite. The NGPs contain single-layer graphene or multi-layer graphene sheets having a thickness from 0.335 nm to 100 nm, and the NGPs occupy a weight fraction of 1% to 99.9% of the total composite weight. The graphene oxide binder, having an oxygen content of 1-40% by weight based on the total graphene oxide weight, occupies a weight fraction of 0.1% to 99% of the total composite weight. The composite forms a thin film with a thickness no greater than 1 mm, preferably thinner than 200 μm, and further preferably not greater than 100 μm. The film thickness is preferably thicker than 10 μm, but can be thinner. The multi-layer graphene sheets typically and preferably have a thickness of 3.35 nm to 33.5 nm and the film has a thickness preferably between 10 and 100 μm.

Preferably, the graphene oxide binder has an oxygen content of 1-10% by weight based on the total graphene oxide weight. The graphene oxide binder preferably occupies a weight fraction of 1% to 40% of the total composite weight. The graphene oxide may be obtained from a graphene oxide gel. This gel is obtained by immersing a graphitic material in a powder or fibrous form in a strong oxidizing liquid in a reaction vessel at a reaction temperature for a length of time sufficient to obtain a graphene oxide gel.

This graphene oxide gel has the characteristics that it is optically transparent or translucent and visually homogeneous with no discernible discrete graphene or graphene oxide sheets dispersed therein. In contrast, conventional suspension of discrete graphene or graphene oxide sheets, or graphite flakes looks opaque, dark, black or heavy brown in color with individual graphene, graphene oxide sheets, or graphite flakes being discernible or recognizable with naked eyes.

The graphene oxide molecules dissolved in a graphene oxide gel are aromatic chains that have an average number of benzene rings in the chain typically less than 1000, more typically less than 500, and most typically less than 100. Most of the molecules have more than 5 or 6 benzene rings (mostly >10 benzene rings) from combined atomic force microscopy, high-resolution TEM, and molecular weight measurements. These benzene-ring type of aromatic molecules have been heavily oxidized and contain functional groups, such as —COOH and —OH and, therefore, are “soluble” (not just dispersible) in polar solvents, such as water.

These soluble molecules behave like polymers and are surprisingly capable of serving as a binder or adhesive that bonds NGPs together to form a composite thin film of good structural integrity and high thermal conductivity. Conventional discrete graphene or graphene oxide sheets do not have any binding or adhesion power.

A preferred embodiment of the present invention is a graphene composite thin film composition that is obtained by mixing NGPs in a graphene oxide gel to form a NGP-graphene oxide mixture suspension, making the suspension into a thin film form, and removing the residual liquid from the mixture suspension. The resulting composite is composed of NGPs that are bonded by a graphene oxide binder to form an essentially pore-free film or an integral film of low porosity level (having a physical density greater than 1.4 g/cm³, more typically >1.6 g/cm³, and often >1.8 g/cm³, approaching the theoretical density (2.25 g/cm³) of graphene sheets.

The graphene oxide binder is made from a graphene oxide gel obtained by dissolving a graphitic material in a fluid containing a strong oxidizing agent at a desired temperature for a length of time sufficient to form a gel. The starting graphitic material for making graphene oxide gel may be selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof. The NGPs may also be produced from a graphitic material selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-beads, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.

The present invention further provides a bulk graphene composite composition composed of nano graphene platelets (NGPs) bonded by a graphene oxide binder obtained from a graphene oxide gel, wherein the NGPs contain single-layer graphene or multi-layer graphene sheets having a thickness from 0.335 nm to 100 nm, and the NGPs occupy a weight fraction of 1% to 99.9% of the total composite weight; and the graphene oxide binder, having an oxygen content of 0.1%-40% by weight based on the total graphene oxide weight, occupies a weight fraction of 0.1% to 99% of the total composite weight.

The present invention also provides a heat spreader or heat sink product for use in a hand-held device, such as a power tool, a microelectronic or telecommunication device (e.g. mobile phone, tablet, laptop computer, LCD display, etc), a light-emitting diode (LED) lighting device or system. The light weight (lower density compared to metal and ceramic materials), exceptional thermal conductivity, and relatively high structural integrity make the invented graphene oxide bonded NGP composite an ideal thermal management material.

The present invention also provides a process for producing a graphene oxide-bonded graphene composite film. The process entails: (a) preparing single-layer or multilayer graphene platelets from a graphitic material; (b) preparing a graphene oxide gel having graphene oxide molecules dispersed in a fluid medium, wherein the graphene oxide gel is optically transparent or translucent; (c) mixing the graphene platelets in the graphene oxide gel to form a composite gel; and (d) forming the composite gel into the composite thin film by removing the fluid medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A flow chart illustrating various prior art processes of producing exfoliated graphite products (flexible graphite sheets and flexible graphite composites) and pyrolytic graphite films, along with presently invented processes of producing a graphene oxide gel-bonded NGP composite.

FIG. 2 (a) A SEM image of a graphite worm sample after exfoliation of graphite intercalation compounds (GICs); (b) An SEM image of a cross-section of a flexible graphite sheet, showing many graphite flakes with orientations not parallel to the flexible graphite sheet surface and many defect structures.

FIG. 3 (A) Thermal conductivity, (B) electrical conductivity, and (C) tensile strength of GO gel bonded NGP composites as a function of the GO content in the composite.

FIG. 4 Thermal conductivity values of GO gel bonded NGP composites and polyimide-based graphitic films as a function of the heat treatment temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Graphite is made up of layer planes of hexagonal networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. These layers of carbon atoms, commonly referred to as graphene layers or basal planes, are weakly bonded together in their thickness direction by weak van der Waals forces and groups of these graphene layers are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. These anisotropic structures give rise to many properties that are highly directional, such as thermal and electrical conductivity.

The graphite structure is usually characterized in terms of two axes or directions: the “c” axis or direction and the “a” axes or directions. The “c” axis is the direction perpendicular to the basal planes. The “a” axes are the directions parallel to the basal planes (perpendicular to the “c” direction). The graphite suitable for manufacturing flexible graphite sheets is typically natural graphite flakes that possess a high degree of orientation.

Due to the weak van der Waals forces holding the parallel graphene layers, natural graphite can be treated so that the spacing between the graphene layers can be appreciably opened up so as to provide a marked expansion in the “c” direction, and thus form an expanded graphite structure in which the laminar character of the carbon layers is substantially retained. The process for manufacturing flexible graphite is well-known and the typical practice is described in U.S. Pat. No. 3,404,061 to Shane et al., the disclosure of which is incorporated herein by reference. In general, flakes of natural graphite are intercalated in an acid solution to produce graphite intercalation compounds (GICs). The GICs are washed, dried, and then exfoliated by exposure to a high temperature for a short period of time. This causes the flakes to expand or exfoliate in the “c” direction of the graphite up to 80-300 times of their original dimensions. The exfoliated graphite flakes are vermiform in appearance and, hence, are commonly referred to as worms. These worms of graphite flakes which have been greatly expanded can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as “flexible graphite”) having a typical density of about 0.04-2.0 g/cm³ for most applications.

The upper left portion of FIG. 1 shows a flow chart that illustrates the prior art processes used to fabricate flexible graphite and the resin-impregnated flexible graphite composite. The processes typically begin with intercalating graphite particles 20 (e.g., natural graphite or synthetic graphite flakes) with an intercalant (typically a strong acid or acid mixture) to obtain a graphite intercalation compound 22 (GIC). After rinsing in water to remove excess acid, the GIC becomes “expandable graphite.” The GIC or expandable graphite is then exposed to a high temperature environment (e.g., in a tube furnace preset at a temperature in the range of 800-1,050° C.) for a short duration of time (typically from 15 seconds to 2 minutes). This thermal treatment allows the graphite to expand in its “c” direction by a factor of 30 to several hundreds to obtain a worm-like vermicular structure, which contains exfoliated, but unseparated graphite flakes 24 with large pores interposed between these interconnected flakes.

In one prior art process, the exfoliated graphite is re-compressed by using a calendering or roll-pressing technique to obtain flexible graphite sheets or foils 26, which are typically much thicker than 100 μm. It seems that no flexible graphite sheet thinner than 75 μm has ever been reported in the open literature or patent documents. Commercially available flexible graphite sheets normally have a thickness greater than 0.125 mm (125 μm), an in-plane electrical conductivity of 1-3×10³ S/cm, through-plane (thickness-direction) electrical conductivity of 15-30 S/cm, in-plane thermal conductivity of 140-190 W/(mK), and through-plane thermal conductivity of approximately 5 W/(mK). These properties are inadequate for many thermal management applications and the present invention is made to address these issues.

In another prior art process, the exfoliated graphite worm 24 may be impregnated with a resin and then compressed and cured to form a flexible graphite composite 28, which is normally of low strength.

The exfoliated graphite may be subjected to high-intensity mechanical shearing/separation treatments using an air mill, ball mill, or ultrasonic device to produce separated nano-scaled graphene plates 34 (NGPs) with all the graphite platelets thinner than 100 nm, mostly thinner than 10 nm. An NGP is composed of a graphene sheet or a plurality of graphene sheets with each sheet being a two-dimensional, hexagonal carbon structure.

For the purpose of defining the geometry and orientation of an NGP, the NGP is described as having a length (the largest dimension), a width (the second largest dimension), and a thickness. The thickness is the smallest dimension, which is no greater than 100 nm, preferably smaller than 10 nm in the present application. When the platelet is approximately circular in shape, the length and width are referred to as diameter. In the presently defined NGPs, both the length and width can be smaller than 1 μm, but can be larger than 200 μm.

In addition to graphene or NGPs, another ingredient of the presently invented composite thin film composition is graphene oxide (GO), which is obtained from a graphene oxide gel. This gel is obtained by immersing a graphitic material 20 in a powder or fibrous form in a strong oxidizing liquid in a reaction vessel to form an optically opaque suspension or slurry. The reaction between graphite powder and the oxidizing agent is allowed to proceed at a reaction temperature for a length of time sufficient to transform this opaque suspension into a translucent or transparent solution, which is now a homogeneous fluid called “graphene oxide gel.”

This graphene oxide gel is optically transparent or translucent and visually homogeneous with no discernible discrete graphene or graphene oxide sheets dispersed therein. In contrast, conventional suspension of discrete graphene sheets, graphene oxide sheets, and expanded graphite flakes look dark, black or heavy brown in color with individual graphene or graphene oxide sheets or expanded graphite flakes discernible or recognizable with naked eyes.

The graphene oxide molecules dissolved in a graphene oxide gel are aromatic chains that have an average number of benzene rings in the chain typically less than 1000, more typically less than 500, and most typically less than 100. Most of the molecules have more than 5 or 6 benzene rings (mostly >10 benzene rings) from combined atomic force microscopy, high-resolution TEM, and molecular weight measurements. Based on our elemental analysis, these benzene-ring type of aromatic molecules are heavily oxidized, containing a high concentration of functional groups, such as —COOH and —OH and, therefore, are “soluble” (not just dispersible) in polar solvents, such as water. The estimated molecular weight of these graphene oxide polymers in the gel state is typically between 200 g/mole and 43,000 g/mole, more typically between 400 g/mole and 21,500 g/mole, and most typically between 400 g/mole and 4,000 g/mole.

These soluble molecules behave like polymers and are surprisingly capable of serving as a binder or adhesive that bonds NGPs together to form a composite thin film of good structural integrity and high thermal conductivity. Conventional discrete graphene or graphene oxide sheets do not have any binding or adhesion power.

The present invention provides a graphene composite thin film composition composed of nano graphene platelets (NGPs) bonded by a graphene oxide binder, wherein the NGPs contain single-layer graphene or multi-layer graphene sheets having a thickness from 0.335 nm to 100 nm, and the NGPs occupy a weight fraction of 1% to 99.9% of the total composite weight; and the graphene oxide binder (having an oxygen content of 1-40% by weight based on the total graphene oxide weight) occupies a weight fraction of 0.1% to 99% of the total composite weight, and wherein the composite forms a thin film with a thickness no greater than 1 mm, preferably less than 200 μm, further preferably less than 100 μm. More preferably, the thickness is greater than 10 μm, further preferably between 10 and 100 μm, and most preferably between 10 μm and 50 μm. A thickness less than 10 μm would make it impossible to handle the composite thin film when attempting to incorporate pieces of the composite thin film in a device for thermal management applications (e.g. as a heat spreader in a microelectronic device).

The multi-layer graphene sheets preferably have a thickness of 3.35 nm to 33.5 nm and the resulting composite film has a thickness no greater than 100 μm. When multi-layer graphene sheets have a thickness of 6.7 nm to 20 nm, one can readily produce a composite film having a thickness not greater than 50 μm.

The graphene oxide-bonded graphene composite thin film composition desirably contains pristine graphene containing no oxygen. The pristine graphene can be obtained from direct ultrasonication without involving oxidation of a graphitic material.

The graphene oxide (GO) binder has an oxygen content of 1-10% by weight based on the total graphene oxide weight. The GO binder, when in a gel state, typically has an oxygen content of 20-46% by weight. After combining with NGPs to form a composite thin film, the process naturally reduces the oxygen content to typically 1-10% by weight.

The graphene oxide binder occupies a weight fraction of 1% to 20% of the total composite weight. The graphene oxide is obtained from a graphene oxide gel, which gel is composed of graphene oxide molecules dispersed in an acidic medium having a pH value of no higher than 5 and the graphene oxide molecules have an oxygen content no less than 20% by weight. In particular, the gel is obtained by immersing a graphitic material in a powder or fibrous form in an oxidizing liquid in a reaction vessel at a reaction temperature for a length of time sufficient to obtain a graphene oxide gel composed of graphene oxide molecules dispersed in an acidic medium having a pH value of no higher than 5 and the graphene oxide molecules have an oxygen content no less than 20% by weight. The graphene composite is obtained by mixing the NGPs in the graphene oxide gel to form a NGP-graphene oxide mixture suspension, making the suspension into a thin film form, and removing the residual liquid from the mixture suspension.

The starting graphitic material for the purpose of forming graphene oxide gel may be selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof. The graphitic material is preferably in a powder or short filament form having a dimension lower than 20 μm, more preferably lower than 10 μm, further preferably smaller than 5 μm, and most preferably smaller than 1 μm.

Using artificial graphite with an average particle size of 9.7 μm as an example, a typical procedure involves dispersing graphite particles in an oxidizer mixture of sulfuric acid, nitric acid, and potassium permanganate (at a weight ratio of 3:1:0.05) at a temperature of typically 0-60° C. for typically at least 3 days, preferably 5 days, and more preferably 7 days or longer. The average molecular weight of the resulting graphene oxide molecules in a gel is approximately 20,000-40,000 g/mole if the treatment time is 3 days, <10,000 g/mole if 5 days, and <4,000 g/mole if longer than 7 days. The required gel formation time is dependent upon the particle size of the original graphitic material, a smaller size requiring a shorter time. It is of fundamental significance to note that if the critical gel formation time is not reached, the suspension of graphite powder and oxidizer (graphite particles dispersed in the oxidizer liquid) appears completely opaque, meaning that discrete graphite particles remain suspended (but not dissolved) in the liquid medium. As soon as this critical time is exceeded, the whole suspension becomes optically translucent or transparent, meaning that the heavily oxidized graphite completely lost its original graphite identity and the resulting graphene oxide molecules are completely dissolved in the oxidizer liquid, forming a homogeneous solution (no longer just a suspension or slurry).

It must be further noted that if the suspension or slurry, with a treatment time being shorter than the required gel formation time, is rinsed and dried, we would simply recover a graphite oxide powder or graphite intercalation compound (GIC) powder, which can be exfoliated and separated to produce nano graphene platelets (NGPs).

Hence, the NGPs may be produced by subjecting a graphitic material to a combined treatment of oxidation, exfoliation, and separation. This graphitic material may also be selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof. The NGPs can also be produced from a process such as (a) direct ultrasonication, (b) potassium melt intercalation and water/alcohol-induced exfoliation, or (c) supercritical fluid intercalation/exfoliation/separation of non-oxidized graphitic material. These processes produce pristine graphene that contains no oxygen.

The graphene composite thin film composition typically has a thermal conductivity greater than 800 Wm⁻¹K⁻¹, more typically greater than 1,000 Wm⁻¹K⁻¹ (even when the film thickness is greater than 10 μm) and often greater than 1,700 Wm⁻¹K⁻¹. The composite thin film has an electrical conductivity greater than 3,000 S/cm. This high electrical conductivity (greater than 3000 S/cm) can be achieved concurrently with a thermal conductivity greater than 1,000 Wm⁻¹K⁻¹. Quite often, the composite thin film can exhibit a combination of a high electrical conductivity (greater than 1,500 S/cm), a high thermal conductivity (greater than 600 Wm⁻¹K⁻¹), a relatively high physical density (greater than 1.4 g/cm³), and a relatively high tensile strength (greater than 10 MPa).

Quite surprisingly, in many samples, the composite thin film has an electrical conductivity greater than 2,000 S/cm, a thermal conductivity greater than 800 Wm⁻¹K⁻¹, a physical density greater than 1.8 g/cm³, and a tensile strength greater than 40 MPa. This combination of superior properties has not been achieved with any graphite or non-graphite material. In some cases, the composite thin film has an electrical conductivity greater than 3,000 S/cm, a thermal conductivity greater than 1,500 Wm⁻¹K⁻¹, a physical density greater than 2.0 g/cm³, and a tensile strength greater than 40 MPa. This type of graphene composite film may be used as a heat spreader component in a portable device.

The present invention also provides a process for producing a graphene oxide-bonded graphene composite film. The process comprises (a) preparing single-layer or multilayer graphene platelets from a graphitic material; (b) preparing a graphene oxide gel having graphene oxide molecules dispersed in an acidic fluid medium; (c) mixing the graphene platelets in the graphene oxide gel to form a composite gel; and (d) forming a composite gel into a composite thin film by removing the fluid medium.

The graphene platelets preferably are pristine graphene containing no oxygen. The pristine graphene is prepared from a graphitic material without involving oxidation of graphite.

As illustrated in FIG. 1, the graphene oxide gel is prepared by immersing a graphitic material in a powder or fibrous form in a strong oxidizing liquid in a reaction vessel at a reaction temperature for a length of time sufficient to obtain a graphene oxide gel composed of graphene oxide molecules dispersed in an acidic medium having a pH value of no higher than 5 and the graphene oxide molecules have an oxygen content no less than 20% by weight. Such a graphitic material is selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.

The graphene platelets may be produced from a graphitic material selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof. The starting material may be immersed in a mixture of sulfuric acid, nitric acid, and potassium permanganate to allow for intercalation of acid into interior of the graphitic, resulting in the formation of graphite intercalation compound (GIC) after rinsing and drying. The GIC is then exfoliated in a high temperature furnace to obtain exfoliated graphite, which is then subjected to mechanical shearing to obtain isolated NGPs.

For the preparation of graphene oxide gel-bonded graphene composite thin film, the discrete NGPs (preferably thinner than 20 nm, more preferably thinner than 10 nm) are dispersed in a graphene oxide gel to produce a suspension wherein discrete graphene platelets (NGPs) are suspended in the oxidizer mixture liquid 36 (FIG. 1), in which graphene oxide molecules are dissolved as well. The graphene platelet concentration is preferably lower than 50% by weight in the suspension (most preferably smaller than 20%). The suspension (or slurry) is allowed to form into thin film structures using techniques like solvent cast, vacuum-assisted filtration, spin coating, dip coating, or paper-making. Upon removal of the liquid medium, the resulting thin-film structures 40 contain discrete graphene platelets being dispersed in a polymer binder matrix (graphene oxide molecules). This thin-film structure is then subjected to a thermal treatment or re-graphitization treatment (typically 100-1000° C., but can be higher), which allows individual graphene oxide molecules to chemically bond to graphene sheets or platelets. This thermal treatment also surprisingly enables or activates the re-joining, polymerization, or chain-growth of otherwise small graphene oxide molecules, resulting in removal of non-carbon elements (e.g. H and O) and formation of large graphene sheets. The presence of discrete NGPs provides “nucleation sites” to accelerate the growth of these huge graphene sheets. It appears that the original NGPs (discrete graphene platelets) and graphene oxide molecules can be merged and integrated into one unitary entity that exhibits an unprecedented combination of exceptional thermal conductivity, electrical conductivity, structural integrity (strength and ease of handling). These properties are unmatched by any graphitic or non-graphitic materials.

The thermal treatment process can be assisted with a calendering or roll-pressing operation to help improve the surface finish of the resulting thin film. The film thickness can be less than 10 μm, but preferably between 10 μm and 100 μm.

Thus, the present invention also provides a process for producing graphene-oxide-bonded pristine graphene composite 42 (FIG. 1), which involves mixing discrete NGPs in said graphene oxide gel to form a NGP-graphene oxide mixture suspension 38, making the suspension into a thin film form, removing a residual liquid from the mixture suspension, and subjecting the thin film 42 to a re-graphitization treatment at a temperature in the range of 100° C. and 3,200° C. The re-graphitization temperature is preferably in the range of 300° C. and 1,500° C. or in the range of 300° C. and 1,000° C. The thin film composition can form into a unitary structure after the re-graphitization treatment.

As indicated above, flexible graphite sheets prepared by re-compression of exfoliated graphite or graphite worms exhibit relatively low thermal conductivity and mechanical strength. The graphite worms can be formed into flexible graphite sheets by compression, without the use of any binding material, presumably due to the mechanical interlocking between the voluminously expanded graphite flakes. Although a significant proportion of these flakes are oriented in a direction largely parallel to the opposing surfaces of a flexible graphite sheet (as evidenced by the high degree of anisotropy with respect to thermal and electrical conductivity), many other flakes are distorted, kinked, bent over, or oriented in a direction non-parallel to these sheet surfaces. This observation has been well demonstrated in many scanning electron micrographs (SEM) published in open or patent literature. Furthermore, the presence of a large number of graphite flakes implies a large amount of interface between flakes, resulting in very high contact resistance (both thermal and electrical resistance).

As a consequence, the electrical or thermal conductivity of the resulting flexible graphite sheets dramatically deviates from what would be expected of a perfect graphite single crystal or a graphene layer. For instance, the theoretical in-plane electrical conductivity and thermal conductivity of a graphene layer are predicted to be 1-5×10⁴ S/cm and 3,000-5,000 W/(mK), respectively. However, the actual corresponding values for flexible graphite are 1-3×10³ S/cm and 140-300 W/(mK), respectively; one order of magnitude lower than what could be achieved. By contrast, the corresponding values for the presently invented graphene-oxide bonded graphene composite films are 3.5-10×10³ S/cm and 600-2,230 W/(mK), respectively.

The present invention also provides a highly thermally conductive NGP-GO composite thin-film sheet that can be used for thermal management applications; e.g. for use as a heat spreader in a microelectronic device (such as mobile phone, notebook computer, e-book, and tablet), flexible display, light-emitting diode (LED), power tool, computer CPU, and power electronics. We are filing separate patent applications to claim the various products or applications of the presently invented NGP-GO composite thin-films.

Example 1 Preparation of Nano Graphene Platelets (NGPs)

Chopped graphite fibers with an average diameter of 12 μM was used as a starting material, which was immersed in a mixture of concentrated sulfuric acid, nitric acid, and potassium permanganate (as the chemical intercalate and oxidizer) to prepare graphite intercalation compounds (GICs). The fiber segments were first dried in a vacuum oven for 24 h at 80° C. Then, a mixture of concentrated sulfuric acid, fuming nitric acid, and potassium permanganate (at a weight ratio of 4:1:0.05) was slowly added, under appropriate cooling and stirring, to a three-neck flask containing fiber segments. After 16 hours of reaction, the acid-treated graphite fibers were filtered and washed thoroughly with deionized water until the pH level of the solution reached 6. After being dried at 100° C. overnight, the resulting graphite intercalation compound (GIC) was subjected to a thermal shock at 1050° C. for 45 seconds in a tube furnace to form exfoliated graphite (worms). Five grams of the resulting exfoliated graphite (EG) were mixed with 2,000 ml alcohol solution consisting of alcohol and distilled water with a ratio of 65:35 for 12 hours to obtain a suspension. Then the mixture or suspension was subjected to ultrasonic irradiation with a power of 200 W for various times. After two hours of sonication, FG particles were effectively fragmented into thin NGPs. The suspension was then filtered and dried at 80° C. to remove residue solvents. The as-prepared NGPs have an average thickness of approximately 9.7 nm.

Example 2 Preparation of Single-Layer Graphene from Meso-Carbon Micro-Beads (MCMBs)

Meso-carbon microbeads (MCMBs) were supplied from China Steel Chemical Co. This material has a density of about 2.24 g/cm³ with a median particle size of about 16 μm. MCMB (10 grams) were intercalated with an acid solution (sulfuric acid, nitric acid, and potassium permanganate at a ratio of 4:1:0.05) for 72 hours. Upon completion of the reaction, the mixture was poured into deionized water and filtered. The intercalated MCMBs were repeatedly washed in a 5% solution of HCl to remove most of the sulphate ions. The sample was then washed repeatedly with deionized water until the pH of the filtrate was neutral. The slurry was dried and stored in a vacuum oven at 60° C. for 24 hours. The dried powder sample was placed in a quartz tube and inserted into a horizontal tube furnace pre-set at a desired temperature, 1,080° C. for 45 seconds to obtain a graphene material. TEM and atomic force microscopic studies indicate that most of the NGPs were single-layer graphene.

Example 3 Preparation of Pristine Graphene

In a typical procedure, five grams of graphite flakes, ground to approximately 20 μm or less in sizes, were dispersed in 1,000 mL of deionized water (containing 0.1% by weight of a dispersing agent, Zonyl® FSO from DuPont) to obtain a suspension. An ultrasonic energy level of 85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation, and size reduction of graphene sheets for a period of 15 minutes to 2 hours.

Example 4 Preparation of Graphene Oxide Gel

Graphite oxide gel was prepared by oxidation of graphite flakes with an oxidizer liquid consisting of sulfuric acid, sodium nitrate, and potassium permanganate at a ratio of 4:1:0.05 at 30° C. When natural graphite flakes (particle sizes of 14 μm) were immersed and dispersed in the oxidizer mixture liquid, the suspension or slurry appears optically opaque and dark. The suspension remains opaque during the first 52 hours of reaction. However, the suspension gradually turns optically translucent (a little cloudy) when the reaction time exceeds 52 hours, and the color of the suspension changes anti black to dark brown. After 96 hours, the suspension suddenly becomes an optically transparent solution with light brown color. The solution appears very uniform in color and transparency, indicating the absence of any dispersed discrete objects. The whole solution behaves like a gel, very similar to a typical polymer gel.

Surprisingly, by casting this gel on a glass surface and removing the liquid medium from the cast film we obtain a thin film of graphene oxide that is optically transparent. This thin film looks like and behaves like a regular polymer film.

Example 5 Preparation of Graphene Oxide Bonded Graphene Composite Thin-Film

The NGPs prepared in Examples 1-3 and the graphene oxide gel prepared in Example 4 were used for the preparation of graphene oxide-bonded graphene composite. Fully separated NGP platelets were dispersed in graphene oxide gel to produce a graphene platelet suspension with the platelet concentration of approximately 1-50% by weight (preferably 5-20% by weight NGP). Ultrasonic waves were employed to assist in the dispersion of NGPs in the gel. This NGP-gel suspension or slurry was then cast onto a glass surface and regulated by a doctor's blade to form a film of uniform thickness. The liquid in the film was then removed in a vacuum oven to form a solid composite film.

Some selected solid films were subjected to a heat treatment (re-graphitization treatment) at a temperature of 100-1,000° C. (in some cases, 1,500-2,800° C., for comparison purposes). For comparison, we also carbonized polyimide films at 1000° C. for 3 hours in an inert atmosphere and then graphitized the films at a temperature in the range of 2,500-3,000° C. for 5 hours to form a conventional graphitic film. Flexible graphite sheets were also obtained from commercial sources as another baseline material.

The in-plane thermal and electrical conductivities and tensile properties of various films were investigated. Several significant observations can be made from the testing results (e.g. as summarized in FIG. 3(A), (B), (C) and FIG. 4):

-   -   (1) At a thickness of approximately 45 μm the thermal         conductivity of the graphene oxide-bonded NGP composite films         (heat treated at 800° C.) increases from 720 W/(mK) at 0% GO         binder (containing graphene sheets only) to reach a maximum of         1310 W/(mK) at 30% GO, as shown in FIG. 3(A). The thermal         conductivity value begins to decrease with a further increase in         the GO binder amount. This maximum thermal conductivity is         significantly higher than that (720 W/mK) of the thin film made         up of NGPs only (0% GO) and that of the film containing GO only         (100% GO). These data have clearly demonstrated an un-expected,         synergistic effect between NGP (graphene) and GO (graphene         oxide).     -   (2) These graphene-based composite thin films exhibit much         higher thermal conductivity values than those (typically 140-300         W/(mK)) of commercially available flexible graphite sheets.     -   (3) FIG. 3(B) also shows a dramatic synergistic effect in         electrical conductivity when graphene is combined with graphene         oxide to form a composite material.     -   (4) Another unexpected observation is the notion that the         tensile strength of the GO-NGP composite increases monotonically         with the increasing GO content. This appears to suggest that GO         gel has a strong adhering power capable of bonding graphene         sheets or integrating with graphene sheets together, and that GO         molecules in a GO gel are capable of combining with one another         to form larger and stronger graphene/graphene oxide sheets that         are relatively defect-free, leading to a relatively high         mechanical strength.     -   (5) The presently invented GO-NGP composite can be readily         produced into thin films much thinner than 100 μm (the practical         lower limit of flexible graphite sheet thickness). Within the         thickness range of 10-100 μm, the GO NGP composite films exhibit         an exceptionally high thermal conductivity of >1800 W/(mK) and         the thermal conductivity is relatively independent of the film         thickness (1820 W/mK at 24 μm, 1831 W/mK at 54 μm, and 1826 W/mK         at 72 μm in thickness). It may be noted that NGPs alone (without         GO gel as a binder), when packed into a film or paper sheet of         non-woven aggregates, exhibit a thermal conductivity higher than         1,000 W/mK only when the film or paper is cast and pressed into         a sheet having a thickness lower than 10 μm, and higher than         1,500 W/mK only when the film or paper is cast and pressed into         a sheet having a thickness lower than 1 μm. This was reported in         our earlier U.S. patent application Ser. No. 11/784,606 (Apr. 9,         2007). However, ultra-thin film or paper sheets (<10 μm) are         difficult to produce in mass quantities, and difficult to handle         when one tries to incorporate these thin films as a heat         spreader material during the manufacturing of microelectronic         devices. Further, thickness dependence of thermal conductivity         (not being able to achieve a high thermal conductivity at a wide         range of film thicknesses) is not a desirable feature.     -   (6) FIG. 3(A) also demonstrates that GO-bonded pristine graphene         composite exhibits significantly higher thermal conductivity as         compared to GO-bonded non-pristine NGP composites. The NGPs         prepared from chemical oxidation or intercalation processes tend         to have structural defects on graphene plane and have some         non-carbon elements (e.g. oxygen and hydrogen). These NGPs are         distinct from pristine NGPs (e.g. prepared from direct         ultrasonication of graphite, alkali metal intercalation, and         supercritical fluid extraction) in terms of chemical         composition, microstructure, and properties).     -   (7) As indicated in FIG. 4, the presently invented NGP-GO         composite compositions do not have to go through an         ultra-high-temperature graphitization treatment. Graphitization         of a carbonized resin (e.g. polyimide) or other carbon materials         requires a temperature typically higher than 2,000° C., most         typically higher than 2,500° C. The graphitization temperature         is most typically in the range of 2,800-3,200° C. in order for         carbonized materials or pyrolytic graphite to achieve a thermal         conductivity >1,600 W/mK. In contrast, the typical heat         treatment temperature (re-graphitization treatment) of the         presently invented NGP-GO composites is significantly lower than         1,500° C. and more typically lower than 1,000° (can be as low as         100° C.). For instance, carbonized polyimide, if graphitized at         2,000° C. for 5 hours, exhibits a thermal conductivity of 820         W/mK. In contrast, we were able to reach a thermal conductivity         of 876 W/mK with a heat treatment of NGP-GO at 500° C. for two         hours. This is very surprising and no one has ever thought that         such a low graphitization temperature was possible. Clearly,         this is a dramatically faster, less energy-intensive, and more         cost-effective process.

In conclusion, we have successfully developed a new and novel class of highly conducting graphene oxide gel-bonded composites that contain non-pristine or pristine graphene sheets. The thermal and electrical conductivities and tensile strength exhibited by the presently invented materials are much higher than what prior art flexible graphite sheets or other graphitic films could achieve. The thermal and electrical conductivities exhibited by the presently invented materials are the highest of what graphite-type thin-layer materials (>10 μm) could achieve. 

We claim:
 1. A graphene composite thin film composition composed of nano graphene platelets (NGPs) that are dispersed in a polymer matrix of graphene oxide and bonded by a graphene oxide binder, wherein said NGPs contain single-layer graphene or multi-layer graphene sheets having a thickness from 0.335 nm to 100 nm, and said NGPs occupy a weight fraction of 1% to 99.9% of the total composite weight; and said graphene oxide binder, having an oxygen content of 0.1%-40% by weight based on the total graphene oxide weight, occupies a weight fraction of 0.1% to 99% of the total composite weight, and wherein said composite contains NGPs that are chemically bonded together by graphene oxide molecules and said composite contains chemically merged graphene oxide molecules and forms a thin film that is essentially pore-free and has a physical density greater than 1.4 g/cm³ and a thickness no greater than 1 mm, and said thin film has an in-plane thermal conductivity greater than 600 Wm⁻¹K⁻¹ and an in-plane electrical conductivity greater than 2,000 S/cm.
 2. The graphene composite thin film composition as defined in claim 1, which has a thickness less than 100 μm.
 3. The graphene composite thin film composition as defined in claim 1, which has a thickness greater than 10 μm, but less than 100 μm.
 4. The graphene composite thin film composition as defined in claim 1 wherein said multi-layer graphene sheets have a thickness of 6.7 nm to 20 nm or said film has a thickness not greater than 50 μm.
 5. The graphene composite thin film composition as defined in claim 1, wherein said nano graphene platelets are pristine graphene containing no oxygen.
 6. The graphene composite thin film composition as defined in claim 1, wherein said nano graphene platelets are pristine graphene containing no oxygen and said pristine graphene is obtained from an oxidation-free procedure selected from direct ultrasonication, supercritical fluid intercalation, or alkali metal intercalation.
 7. The graphene composite thin film composition as defined in claim 1 wherein said graphene oxide binder has an oxygen content of 1-10% by weight based on the total graphene oxide weight which is measured after said graphene composite film composition is made.
 8. The graphene composite thin film composition as defined in claim 1 wherein said graphene oxide binder occupies a weight fraction of 1% to 40% of the total composite weight.
 9. The graphene composite thin film composition as defined in claim 1 wherein said graphene oxide is obtained from a graphene oxide gel, which gel is composed of graphene oxide molecules dispersed in an acidic medium having a pH value of no higher than 5 and said graphene oxide molecules have an oxygen content no less than 20% by weight while in a gel state.
 10. The graphene composite thin film composition as defined in claim 1 wherein said graphene oxide is obtained from a graphene oxide gel, which gel is obtained by immersing a graphitic material in a powder or fibrous form in an oxidizing liquid medium in a reaction vessel at a reaction temperature for a length of time sufficient to obtain a graphene oxide gel composed of graphene oxide molecules dispersed in the liquid medium and said graphene oxide molecules have an oxygen content no less than 20% by weight and a molecular weight less than 43,000 g/mole while in a gel state.
 11. The graphene composite thin film composition as defined in claim 10 wherein said graphene oxide molecules have a molecular weight less than 4,000 g/mole.
 12. The graphene composite thin film composition as defined in claim 10 wherein said graphene oxide molecules have a molecular weight between 200 g/mole and 4,000 g/mole.
 13. The graphene composite thin film composition as defined in claim 10 wherein said graphene composite is obtained by mixing said NGPs in said graphene oxide gel to form a NGP-graphene oxide mixture suspension, making said suspension into a thin film form, and removing a residual liquid from said mixture suspension.
 14. The graphene composite thin film composition as defined in claim 10 wherein said graphene composite is obtained by mixing said NGPs in said graphene oxide gel to form a NGP-graphene oxide mixture suspension, making said suspension into a thin film form, removing a residual liquid from said mixture suspension, and subjecting to a re-graphitization treatment at a temperature in the range of 100° C. and 3,200° C.
 15. The graphene composite thin film composition as defined in claim 14 wherein said re-graphitization temperature is in the range of 300° C. and 1,500° C.
 16. The graphene composite thin film composition as defined in claim 14 wherein said re-graphitization temperature is in the range of 100° C. and 1,000° C.
 17. The graphene composite thin film composition as defined in claim 14 wherein said thin film composition forms into a unitary structure after said re-graphitization treatment.
 18. The graphene composite thin film composition as defined in claim 10 wherein said graphitic material is selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.
 19. The graphene composite thin film composition as defined in claim 1 wherein said NGPs are produced from a graphitic material selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.
 20. The graphene composite thin film composition as defined in claim 1 wherein said thin film has a thermal conductivity greater than 1,000 Wm⁻¹K⁻¹.
 21. The graphene composite thin film composition as defined in claim 1 wherein said thin film has a thermal conductivity greater than 1,700 Wm⁻¹K⁻¹.
 22. The graphene composite thin film composition as defined in claim 1 wherein said thin film has an electrical conductivity greater than 3000 S/cm.
 23. The graphene composite thin film composition as defined in claim 1 wherein said thin film has an electrical conductivity greater than 3000 S/cm and a thermal conductivity greater than 1,000 Wm⁻¹K⁻¹.
 24. The graphene composite thin film composition as defined in claim 1 wherein said thin film has an electrical conductivity greater than 1,500 S/cm, a thermal conductivity greater than 600 Wm⁻¹K⁻¹, a physical density greater than 1.4 g/cm3, and a tensile strength greater than 10 MPa.
 25. The graphene composite thin film composition as defined in claim 1 wherein said thin film has an electrical conductivity greater than 2,000 S/cm, a thermal conductivity greater than 800 Wm⁻¹K⁻¹, a physical density greater than 1.8 g/cm3, and a tensile strength greater than 40 MPa.
 26. The graphene composite thin film of claim 1, having an electrical conductivity greater than 3,000 S/cm, a thermal conductivity greater than 1,500 Wm⁻¹K⁻¹, a physical density greater than 2.0 g/cm³, and a tensile strength greater than 40 MPa.
 27. A graphene composite composition composed of nano graphene platelets (NGPs) that are dispersed in a polymer matrix of graphene oxide and bonded by a graphene oxide binder obtained from a graphene oxide gel, wherein said NGPs contain single-layer graphene or multi-layer graphene sheets having a thickness from 0.335 nm to 100 nm, and said NGPs occupy a weight fraction of 1% to 99.9% of the total composite weight; and said graphene oxide binder, having an oxygen content of 0.1%-40% by weight based on the total graphene oxide weight, occupies a weight fraction of 0.1% to 99% of the total composite weight, and wherein said composite contains NGPs that are chemically merged graphene oxide molecules and forms a thin film that is essentially pore-free and has a physical density greater than 1.4 g/cm³ and a thickness no greater than 1 mm, and said thin film has an in-plane thermal conductivity greater than 600 Wm⁻¹ K⁻¹ and an in-plane electrical conductivity greater than 2,000 S/cm.
 28. A process for producing a graphene composite film of claim 1, said process comprising: (a) preparing single-layer or multilayer graphene platelets from a graphitic material; (b) preparing a graphene oxide gel having graphene oxide molecules dispersed in a fluid medium, wherein said graphene oxide gel is optically transparent or translucent; (c) mixing said graphene platelets in said graphene oxide gel to form a composite gel; and (d) forming said composite gel into said composite thin film by removing said fluid medium.
 29. The process of claim 28, wherein said graphene platelets are pristine graphene containing no oxygen.
 30. The process of claim 28, wherein said graphene platelets are pristine graphene containing no oxygen and said pristine graphene is prepared from a graphitic material without involving oxidation.
 31. The process of claim 28, wherein said graphene oxide gel is prepared by immersing a graphitic material in a powder or fibrous form in an oxidizing liquid to form an optically opaque suspension in a reaction vessel at a reaction temperature for a length of time sufficient to obtain a graphene oxide gel that is optically transparent or translucent, wherein said graphene oxide gel is composed of graphene oxide molecules dispersed in an acidic medium having a pH value of no higher than 5 and said graphene oxide molecules have an oxygen content no less than 20% by weight.
 32. The process of claim 28, wherein said graphene platelets are produced from a graphitic material selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.
 33. The process of claim 28, wherein said graphene oxide gel is prepared by immersing a graphitic material in an oxidizing agent to form an optically opaque suspension and allowing an oxidizing reaction to proceed until an optically transparent or translucent solution is formed, and wherein said graphitic material is selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof. 